The present invention relates, in general, to an electromotive drive for operating a member to execute a linear movement and a rotary movement, and more particularly to an electromotive adjustment drive for an injection unit of an injection molding machine to rotate a plasticizing screw for carrying out the plasticizing phase and to linearly move the plasticizing screw for carrying out the injection phase.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
U.S. Pat. No. 5,645,868 describes a drive apparatus for an injection unit, including a hollow-shaft motor which is axially aligned with a spindle drive comprised of a ball screw with a ball nut. The drive apparatus uses a single electric motor and three one-way clutches to provide both the injection and plasticizing functions. Two one-way clutches are engaged to prevent a rotation of the plasticizing screw and to couple the spindle drive with the electric motor, when the plasticizing screw is intended to execute the injection function, whereas the third one-way clutch operatively connects the electric motor directly with the plasticizing screw, while the other one-way clutches slip, when the plasticizing screw is intended to execute the plasticizing function. This single motor drive requires the provision of an auxiliary motor to control the back pressure of the plastic melt during the plasticizing function. In addition, except for the ball nut, all drive components of the drive apparatus, including both motors and the ball screw, associated bearings, and the one-way clutches, must be moved axially together with the plasticizing screw in relation to the extruder barrel so that the overall mass being moved and supported is very large.
It would therefore be desirable and advantageous to provide an improved electromotive adjustment drive to obviate prior art shortcomings and to significantly reduce power consumption as well as to significantly decrease the stress to which bearings are subjected during operation of the member being moved and rotated.